The twin chain drive comprises two parallel chain loops of the same length arranged to run in synchronism over synchronised sprockets. Steps or pallets are fixed between the two chain loops to corresponding points on each loop. The loops pass around, and are driven in synchronism by driving the sprockets. The chain loops carry the load of the escalator and are usually supported in space by wheels, located on each side of the steps, which run on side-tracks to reduce frictional losses. These wheels are usually mounted at each end of an axle that is fixed either to, or through, corresponding points on each of the two chains. The through-axle is present to resist large cyclic bending forces imposed on the axle by couples determined by the distance between chain centre and its corresponding wheel centre, without subjecting the steps or pallets to fatigue-inducing loading. These couples occur when a particular section of chains, axles and wheels run over any area of the machine that causes the chains to be pulled either away from or toward the wheel running tracks. In this design, the wheels take the entire load carried by the escalator and transverse loads are transferred directly to the chains.
In an alternative design, the wheels are replaced by rollers that are rotatably mounted on the pins of each link of the chains, and the rollers run on tracks or flat surfaces. In this design, the entire load carried by the escalator is taken by the rollers and hence by the chain links. Escalators having twin chain drives suffer from a phenomenon known as “lateral float” by which the steps exhibit a strong tendency to drive themselves into the side panels of the escalator adjacent to the steps, known as skirt panels. This damages the skirt panels, the edges of steps, and often the wheels associated with steps, or the rollers.
It is extremely important to ensure that the steps always remain in perfect alignment and that the steps do not creep sideways and contact the side structure of the escalator. In addition, usually at the top and bottom of an escalator, where the steps meet a horizontal floor surface, there is provided a comb that comprises fingers that match corresponding grooves in each step. Therefore, the steps have to be in perfect alignment with the combs at all times, otherwise the escalator will jam.
One cause of lateral float is a result of one chain stretching slightly more than the other chain. This can happen, for example, in those applications, such as on an underground transportation system, where persons using the escalator stand on one side of each step of the escalator to allow persons to pass by walking up or down the moving escalator on the other side of the steps. The chain on the side where people stand carries a much higher load than the other chain, and over time the more heavily loaded chain can become stretched more than the other chain.
In these circumstances, the alignment of the steps can become distorted causing one edge of the steps to advance ahead of the other edge of the step. This imposes a lateral force on the steps causing them to collide with the fixed side skirt panels. This not only causes damage to the steps, skirt panels and the chains but it also throws the whole run of the escalator out so that the steps jam in the combs at the top and bottom.
Attempts have been made to provide lateral constraint of the steps. In one design, the wheels at each side of the steps are arranged to run in guides at each side of the steps. In an alternative arrangement, the wheels are dispensed with, and the rotatable rollers that are mounted on the pins of each link of the chains have a “V” shaped or channelled profile. The rollers are arranged to run with the channelled profile running on guide tracks so that each chain is constrained laterally. These solutions are not very practical because of the very high loads that the wheels or rollers carry and the high side loads imposed on the chains, and have been found to accelerate wear of the chains. Furthermore, such solutions do not allow one chain to stretch slightly more than the other does without causing the wheels or rollers to try to steer away from a straight-line direction. This increases the lateral forces.
An object of the present invention is to provide an escalator of a twin chain drive type with means for providing lateral constraint of the steps that will tolerate slight misalignment of the chains without causing unacceptable misalignment of the steps.